1. Field of the Invention
The present invention relates to an optical glass suitable for using for an optical element, such as a lens, a prism or the like, or a substrate. In particular, it relates to an optical glass having a high refractive index, which is suitable for using for an optical element in a polarization optical system, or for a light polarization control element, such as a polarization beam splitter (hereinafter it is called PBS), a spatial light modulation element (hereinafter it is called SLM), a polarization conversion element or the like.
2. Description of Related Art
An optical system utilizing polarization, that is, a polarization optical system, is utilized for various optical instruments, such as a liquid crystal projector or the like. For example, recently, in a projection device, such as the liquid crystal projector or the like, has been made to have high luminance. However, there is a problem that a transmittance of an optical glass used in each portion in the optical system of the projection device deteriorates with time,
When a high luminance light having large irradiance is irradiated to an optical glass, for example, an undesirable coloring phenomenon, such as solarization, is often caused, so that the transmittance of the glass deteriorates. Solarization generally points the coloring phenomenon of a glass caused by irradiating a light with a wavelength in an ultraviolet region. However, the coloring phenomenon caused by irradiating a light is reported not only as solarization but also as various coloring phenomenon including the coloring phenomenon of a glass caused by two-photon absorption process with visible light caused when a light with a wavelength in a visible range is irradiated to the glass. In particular, when a light with a wavelength in a visible range is irradiated to a borosilicate glass including lead, it is known that coloring of the glass is easily caused by two-photon absorption. As described above, even though there are various wavelengths of the irradiating light that induce the coloring phenomenon, or various mechanism for causing the coloring phenomenon, deterioration of transmittance of the glass is caused as a result of the coloring phenomenon of the glass caused by irradiating a light.
In order to prevent these various coloring phenomenon, means such as incorporating a filter for cutting a light with a harmful wavelength, which becomes a cause of deterioration of luminance (output) of a light source or a cause of the coloring phenomenon, into the optical system or the like, are considered. However, in the optical system that high luminance is required (for example, an optical system of a high luminance liquid crystal projector), those means are not desirable since those are directly related to deterioration of an amount of an output light.
Further, in the projection device such as the liquid crystal projector or the like, in order to project a high luminance light without loosing its color rendering, the optical glass for using for the optical system is required to have an excellent transmittance to a ray with a wavelength from the long wavelength side to the purple region of the visible range.
Moreover, generally, the optical system of the projection device, such as the liquid crystal projector or the like, is a polarization optical system. It is required to control properties of polarization in high accuracy. Among optical parts of the polarization optical system, when a material having an optical anisotropy is used for a member, such as a prism of a light polarization control element, a substrate or the like, which is required to keep the properties of polarization of the PBS, SLM or the like, a phase difference (optical path difference) between a transmitted principal ray and an extraordinary ray perpendicular to the principal ray is changed as compared with the difference before a principal ray is transmitted through the material. Since the properties of polarization cannot be kept, it is necessary that a material having an optical isotropy should be used for these parts.
Even when optical glasses in earlier technology having an optical isotropy annealed sufficiently and removed strain are used for the optical parts which are required to keep the properties of polarization in the polarization optical system, an optical anisotropy caused by photoelastic effect, that is, a double refraction property, is seen if an absolute value of a photoelastic constant of the glass is large when a mechanical stress or thermal stress is added to those glasses. As a result, there is a problem that it becomes difficult to obtain desired properties of polarization.
The above-described mechanical stress is caused, for example, by joining a material having different coefficient of thermal expansion from a glass to the glass. Further, the above-described thermal stress is caused, for example, by generating heat from peripheral devices, or by generating heat from the glass itself caused by absorption of energy of a transmitted light.
An amount of double refraction which the glass induces by applying these stresses to the glass, can be represented by using the optical path difference. When a (nm) is the optical path difference, d (cm) is the thickness of the glass and F (Pa) is the stress, the following equation (1) holds. The equation (1) means that the more the optical path difference increases, the more the double refraction increases.
xcex4=xcex2xc3x97dxc3x97Fxe2x80x83xe2x80x83(1) 
The proportional constant (xcex2) in the equation (1) is called photoelastic constant. The values thereof vary in type of glass. As shown in the equation (1), when the stress (F) applied to the glass and the thickness (d) of the glass are constant, the smaller the absolute value of the photoelastic constant (xcex2) of the glass is, the shorter the optical path difference (xcex4) is, that is, the smaller the double refraction property is.
In earlier technology, as a material of an optical part in a polarization optical system, mainly, a borosilicate glass, such as an S-BSL7 (a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara.), or other equivalent glasses manufactured by other companies, for example, a BK7 (a trademark of the optical glass manufactured by Schott Glas), or the like, is used since it has an excellent transmittance to a ray with a wavelength from the long wavelength side to the purple region in the visible range, and it is inexpensive and easy to be acquired. However, these optical glasses have a large absolute value of photoelastic constant (xcex2). For example, in the S-BSL7, the refractive index (nd) is 1.52 and the value of xcex2 at e-line (wavelength of 546.07 nm) is 2.79xc3x9710xe2x88x925 nm/cm/Pa. In order to control the polarization properties at higher accuracy in the polarization optical system, as described above, an optical glass having a smaller absolute value of the photoelastic constant (xcex2) is required strongly. Further, an optical glass having a higher refractive index is needed from viewpoint of optical design.
As an optical glass having a high refractive index, which has a small absolute value of a photoelastic constant (xcex2), a glass including a large amount of lead has been known since the beginning of the 20th century. A typical glass of SiO2-PbO system manufactured and sold as a glass including a high amount of lead at present, for example, a PBH53 (a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara.), an SF57 (a trade name of the optical glass manufactured by Schott Glas) which has the same refractive index as the PBH53, or the like, is intended to be used in the polarization optical system as a material for controlling the optical properties in higher accuracy, from the above-described reason. For example, in the PBH53, the refractive index (nd) is 1.85 and the photoelastic constant (xcex2) at e-line (wavelength of 546.07 nm) is less than 0.1xc3x9710xe2x88x925 nm/cm/Pa. The PBH53 has a high refractive index, and a sufficiently small absolute value of a photoelastic constant (xcex2) to control the polarization properties, that is, it has a low photoelastic constant.
However, these glasses in earlier technology that include a high amount of lead is bad in a light transmittance from the short wavelength side of the blue region in the visible range to the purple region. For example, in both the PBH 53 and the SF57, the threshold value of the wavelength of a ray which transmits through glasses having a thickness of 10xc2x10.1 mm with a transmittance of 80% including the reflection loss is 440 nm. In the shorter wavelength band than this, the transmittance becomes lower than 80%. When these glasses are used in the polarization optical system, for example, difference in intensities of lights separated in three primary colors, that is, a blue light (B light), a green light (G light), and a red light (R light), in the polarization optical system of a liquid crystal projector or the like, is caused. In order to keep the color rendering of a projected light, in accordance with the blue light (B light) whose intensity is low, the intensities of the other two lights need to be reduced. As a result, there is a problem that the amount of the light projected from a projection device, such as a liquid crystal projector or the like, is not sufficient.
As a glass having a small absolute value of the photoelastic constant (xcex2), other than the above glasses, for example, the Japanese Patent Laid-open Publication No. 7-215732 discloses an optical glass for polarization optical system of SiO2-alkali metal oxide-PbO system. However, this glass shows a high deterioration rate of transmittance (which is the degree of reduction of transmittance before and after transmitting a ray, hereinafter, it may be called an amount of deterioration), Further, also before the deterioration of transmittance is caused, the light transmittance from the short wavelength side of the blue region to the purple region is not sufficient. Moreover, it is difficult to reduce bubbles in a melted glass when the glass is melted and refined. Although it is possible to homogenize the glass by making striae disappear by stirring the melted glass sufficiently, it is not suitable for an optical part since bubbles are remained in the obtained glass.
The Japanese Patent Laid-open Publication No. 9-48631 discloses an optical glass for polarization optical system of PbO-B2O3 and/or Al2O3 system. The amount of deterioration of transmittance in this glass is small, and the light transmittance of this glass is a little better than the glass in the above-described Japanese Patent Laid-open Publication. However, the light transmittance from the short wavelength side of the blue region to the purple region before the deterioration of the transmittance is caused is still not improved sufficiently. Further, bubbles in a melted glass are not reduced sufficiently when the glass is melted and refined. Although it is possible to homogenize the glass by making striae disappear in the same way as the glass in the above-described Japanese Patent Laid-open Publication, it is difficult for using as an optical part since bubbles are remained in the obtained glass.
Further, the Japanese Patent Laid-open Publication No. 8-259259 discloses an optical glass for polarization optical system of SiO2xe2x80x94R2O-PbO-fluorine system. Bubbles in this glass are reduced sufficiently compared with the two glasses in the above-described Japanese Patent Laid-open Publications. However, its amount of deterioration of transmittance is large by far compared with the two glasses in the above-described Japanese Patent Laid-open Publications. This is thought that this is caused by the fluorine component.
Incidentally, an optical glass is generally melted and refined in a container at least a portion of which contacts with a melted glass is made of platinum (a platinum crucible or a tank made of platinum) in order to improve the work efficiency when the glass is melted and refined, and to improve the yield of the glass for manufacturing. In particular, it is general to form a container for refining a glass that needs to be melted at high temperature by platinum.
However, in the PBH53, the SF57, and a glass including a large amount of PbO, such as the glasses in the above-described Japanese Patent Laid-open Publications, particularly, the platinum of a container is melted easily into the glass when the glass is melted and refined. Therefore, the light transmittance tends to be bad.
Therefore, in case of a glass in which bubbles are not reduced sufficiently, such as the glasses in the Japanese Patent Laid-open Publication No. 9-48631 and the Japanese Patent Laid-open Publication No. 8-259259, there is a case that a glass with less or without bubbles can be obtained if the temperature of refining is made high, because the bubbles are reduced sufficiently. However, when the temperature is made high, the light transmittance of the obtained glass becomes worse and worse since the amount of platinum melting in the glass from the container becomes large. On the other hand, when the temperature of refining is made low, the amount of platinum melting in the glass becomes small, and the light transmittance becomes considerably good, however, it is more difficult to reduce the bubbles.
Further, in the glass disclosed in the Japanese Patent Laid-open Publication No. 8-259259, a glass having a good light transmittance can be obtained when the temperature of refining is low. However, the bubbles are not reduced sufficiently, and the amount of deterioration of transmittance becomes large by far since an amount of the fluorine volatilizing from the glass when the glass is refined is small. Further, when the temperature of refining is made high, the bubbles are reduced sufficiently, and the amount of deterioration of transmittance becomes somewhat small since the amount of the fluorine volatilizing from the melted glass becomes large. However, not only the amount of platinum melting in the glass from the container becomes large, but also the light transmittance of the obtained glass becomes extremely bad since the transmittance of the short wavelength band is improved by compounding fluorine in this glass, as disclosed in the same Japanese Patent Laid-open Publication.
An object of the present invention is to solve the above-described various problems in earlier technology. It is to provide a glass suitable for using for an optical element, such an a lens, a prism or the like, or a substrate, in particular, a glass with a high refractive index, which is suitable for using for an optical element in a polarization optical system, or for a light polarization control element, such as a PBS, an SLM, a polarization conversion element or the like, the glass whose amount of deterioration of transmittance caused by irradiating a ray in an ultraviolet range and/or a visible range being small, the glass having an excellent transmittance to a ray with a wavelength from a long wavelength side to a purple region in the visible range, and bubbles in a melted glass when the glass is melted and refined are reduced sufficiently and the glass being excellent in a refining property, and furthermore, the glass having a small absolute value of a photoelastic constant (xcex2) in addition to the above-described various properties.
In order to accomplish the above-described object, the inventors have examined and researched an optical glass. As a result, the inventors have unexpectedly found that a glass having an excellent light transmittance, an amount of deterioration of transmittance thereof being small, and bubbles being reduced sufficiently when the glass is melted and refined, and further to the above properties, the glass having a low photoelastic constant is obtained by adding TeO2 and Li2O to a glass including SiO2, PbO, and B2O3, the glass including SiO2, PbO, and B2O3 having been recognized as a glass having a bad light transmittance in a short wavelength side of a visible range for a long time, and the glass including SiO2, PbO, and B2O3 having been considered that it easily generates coloring by irradiation of a light, that is, deterioration of transmittance. Then, the present invention has been accomplished.
That is, in order to accomplish the above-described object, according to an aspect of the present invention, an optical glass comprises: SiO2; PbO; B2O3; not less than 0.1 mass % of TeO2; and not less than 0.4 mass % of Li2O.
Preferably, in the present invention, a wavelength of a light which transmits through the optical glass with a transmittance of 80% including a reflection loss is not more than 420 nm when a thickness of the glass is 10xc2x10.1 mm.
Here, the transmittance of 80% may be a value including a reflection loss.
Further, in the present invention, a deterioration rate of transmittance of a ray having a wavelength of 450 nm when a ray in at least one of an ultraviolet region and a visible region is irradiated with an irradiance of 2.2 W.cmxe2x88x922 for ten minutes may be not more than 3.0%.
Here a deterioration rate of transmittance is calculated by (T(b)xe2x88x92T(a))/T(b)xc3x97100 when a transmittance of a light with a wavelength of 450 nm before irradiating a light for ten minutes is T(b), and a transmittance of a light with a wavelength of 450 nm after irradiating the light for ten minutes is T(a).
More preferably, in the present invention, a deterioration rate of transmittance of a ray having a wavelength of 450 nm when a ray in at least one of an ultraviolet region and a visible region is irradiated with an irradiance of 2.2 W.cmxe2x88x922 for ten minutes is not more than 1.0%.
Here, a deterioration rate of transmittance is calculated in the same way as the above.
Moreover, in the present invention, an absolute value of a photoelastic constant (xcex2) in a wavelength range of 400 to 800 nm may be not more than 1.0xc3x9710xe2x88x925 nm/cm/Pa.
Furthermore, in the present invention, the optical glass may comprise: 18 to 29 mass % of SiO2; 66 to 78 mass % of PbO; 0.1 to 3.5 mass % of TeO2; 0.1 to 6 mass % of B2O3; and 0.4 to 5 mass % of Li2O. A refractive index (nd) of the glass may be in a range of 1.75 to 1.90.
Preferably, the optical glass may further comprise: 0 to 8 mass % of Na2O; 0 to 8 mass % of K2O; a total amount of Li2O+Na2O+K2O being 0.4 to 10 mass %; 0 to 5 mass % of MgO; 0 to 5 mass % of CaO; 0 to 5 mass % of SrO; 0 to 10 mass % of BaO; 0 to 5 mass % of ZnO; a total amount of MgO+CaO+SrO+BaO+ZnO being 0 to 10 mass %; 0 to 5 mass % of GeO2; 0 to 3 mass % of Al2O3; 0 to 3 mass % of Nb2O5; a total amount of GeO2+Al2O2+Nb2O5 being 0 to 5 mass %; 0 to 1 mass % of As2O3; and 0 to 1 mass % of Sb2O3.
Moreover, in the present invention, the optical glass may comprise: 18 to 29 mass % of SiO2; 66 to 78 mass % of PbO; 0.1 to 3.5 mass % of TeO2; 0.1 to less than 2 mass % of B2O3; 0.4 to 5 mass % of Li2O; 0 to 8 mass % of Na2O; 0 to 8 mass % of K2O; a total amount of Li2O+Na2O+K2O being 0.4 to 10 mass %; 0 to 5 mass % of MgO; 0 to 5 mass % of CaO; 0 to 5 mass % of SrO; 0 to 10 mass % of BaO; 0 to 5 mass % of ZnO; a total amount of MgO+CaO+SrO+BaO+ZnO being 0 to 10 mass %; 0 to 5 mass % of GeO2; 0 to 3 mass % of Al2O3; 0 to 3 mass % of Nb2O5; a total amount of GeO2+Al2O3+Nb2O5 being 0 to 5 mass %; 0 to 1 mass % of As2O3; and 0 to 1 mass % of Sb2O3. A refractive index (nd) may be in a range of 1.75 to 1.90.
Further, in the present invention, an Abbe number (xcexdd) of the optical glass may be less than 28.